Conventionally, various focusing methods have been proposed for small, simple objective lenses suitable for endoscopes (for example, see PTLs 1 to 3). In PTL 1, focusing is achieved by moving some lenses of an objective-lens optical system in the optical axis direction. In PTL 2, a wire is used to move a lens. In PTL 3, removable parallel plates are disposed near an aperture stop, and, by switching between parallel plates having different thicknesses, a shift of the focal position caused by a difference in the wavelength used is corrected.
In a typical focusing method, like that employed in PTL 1, the position of image formation is corrected by moving, in the optical axis direction, a lens having refractive power.
The function of a focusing lens will be described from the standpoint of aberration correction. In an image-forming relationship for a certain object distance, a focusing lens, which is moved for focusing, shares the aberration correction with other lens elements. The ratio of this sharing is determined according to the refractive power, the surface shape, and the height and angle of rays, especially according to the refractive power. In this image-forming relationship, when manufacturing errors, particularly eccentricity errors, such as an error occurring in a direction perpendicular to the optical axis of the lens elements (shifting) or an error associated with tilting of the lens elements (tilting), occur, the sharing ratio of the aberration correction changes, causing asymmetric bokeh or the like, which makes the aberrations worse. The contribution ratio of the focusing lens and the other lens elements to the deterioration of the aberrations is determined according to, in particular, the refractive power, like the sharing ratio of the aberration correction. Hence, if a lens having higher refractive power becomes eccentric, the degree of deterioration of the aberrations tends to be greater.
In the means for moving the lens disclosed in PTL 2, a certain amount of slack has to be provided to facilitate sliding, which leads to eccentricity of the focusing lens. Furthermore, because the point of application of a force with the wire is displaced from the point of action of the lens, the focusing lens tends to become eccentric when moving on the optical axis in the focusing operation (during focusing). In particular, in recent years, with an increase in number of pixels of image sensors, the deterioration of aberrations due to manufacturing errors, such as eccentricity, has become more apparent.
In PTL 3, the focal position is moved not by changing the working distance, but, when observation is performed at a predetermined working distance, by switching the parallel plates having different thicknesses to correct a shift in focal position due to the wavelength used. Therefore, the focal position can only be moved by a very small distance, small enough to correct an axial chromatic aberration of the objective lens.
Objective lenses of endoscopes, which employs the conventional focusing methods, are intended to achieve a wide angle of view with a simple configuration; hence, they have large negative distortion. If focusing is performed with such an objective lens, the image plane is tilted in the minus (under) direction when the working distance changes from the long side to the short side.